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Configuring Feedback Loop Processing

Feedback Loops are provided by several mailbox providers, including AOL, Hotmail, Comcast, and Yahoo! as a method for informing senders regarding which messages are receiving spam complaints.

The mailbox providers send registered senders formatted abuse report messages to a pre-configured address in ARF format, but those messages typically do not include information that can help senders suppress future sends to the recipient that reported the message. KumoMTA can not only process and log ARF messages, but it can also inject tracking headers into the message that it can later decode to preserve recipient data needed for unsubscribing recipients that have reported messages as spam.

For more information on Feedback Loops and how to apply for them, see

Configuring Tracking Headers

By default, KumoMTA will include a supplemental tracking header that will be extracted as part of the ARF message processing. This setting is controlled by the supplemental_header option in the kumo.start_esmtp_listener function. Additional metadata can be preserved by listing the metadata keys desired in the include_meta_names argument.

kumo.start_esmtp_listener {
  -- ..
  trace_headers = {
    -- this is the default: add the supplemental header
    supplemental_header = true,

    -- this is the default: the name of the supplemental header
    header_name = 'X-KumoRef',

    include_meta_names = { 'tenant', 'campaign' },

See the trace headers section of the start_esmtp_listener section of the reference manual for more information.

Configuring ARF Domains

For KumoMTA to process inbound messages as ARF, the inbound receiving domain must be configured as a candidate for ARF processing. This is most simply accomplished using the listener domains policy helper, with a snippet like this:

# accept and log ARF feedback reports sent to
log_arf = true

The preceding example designates that messages injected from remote hosts destined for will be accepted and then processed as ARF abuse report messages.

Message Disposition After Processing

For most use cases, the desired outcome after a message is processed is to discard the message, but in some cases it can be desirable to forward the message for further processing or storage, especially during testing and migration.

To queue a message after processing, add relay_to = true to the domain configuration:

# accept and log ARF feedback reports sent to
log_arf = true
relay_to = true

In addition, it should be noted that the MX record for your domain will still be pointed at the KumoMTA instance, which means that in order to avoid a mail loop you will need to configure routing for the domain to specify where the message should be relayed to from the KumoMTA instance, by overriding the destination queue for the message in the smtp_server_message_received event:

kumo.on('smtp_server_message_received', function(msg)
  if msg:recipient():domain() == '' then
    -- Re-route the message to the intended destination
    msg:set_meta('routing_domain', '[]')

    -- In earlier versions of KumoMTA, you need to set the queue
    -- this way instead
    -- msg:set_meta('queue', '[]')

FBL Message Logs

All feedback loop messages are logged to the destination configured in the configure_local_logs function, using the Feedback type.

The format of a Feedback loop message log entry is as follows:

    "type": "Feedback",
    "feedback_report": {
        "feedback_type": "abuse",
        "user_agent": "SomeGenerator/1.0",
        "version": 1,
        "arrival_date": "2005-03-08T18:00:00Z",
        "incidents": nil,
        "original_envelope_id": nil,
        "original_mail_from": "<>",
        "reporting_mta": {
            "mta_type": "dns",
            "name": "",
        "source_ip": "",
        "authentication_results": [
            "; spf=fail",
        "original_rcpto_to": [
        "reported_domain": [
        "reported_uri": [

        // any fields found in the report that do not correspond to
        // those defined by RFC 5965 are collected into this
        // extensions field
        "extensions": {
            "removal-recipient": [

        // The original message or message headers, if provided in
        // the report
        "original_message": "From: <>
Received: from (
    []) by with ESMTP id M63d4137594e46;
    Tue, 08 Mar 2005 14:00:00 -0400
X-KumoRef: eyJfQF8iOiJcXF8vIiwicmVjaXBpZW50IjoidGVzdEBleGFtcGxlLmNvbSJ9
To: <Undisclosed Recipients>
Subject: Earn money
MIME-Version: 1.0
Content-type: text/plain
Date: Thu, 02 Sep 2004 12:31:03 -0500

Spam Spam Spam
Spam Spam Spam
Spam Spam Spam
Spam Spam Spam

        // if original_message is present, and a kumo-style trace
        // header was decoded from it, then this holds the decoded
        // trace information
        "supplemental_trace": {
            "recipient": "",